1. Field of the Invention
The present invention relates to a paste pattern formation method. More particularly, the present invention relates to a paste pattern formation method for forming a paste pattern having a high aspect ratio. The present invention also relates to a transfer film used in this paste pattern formation method.
2. Description of the Related Art
Various types of patterns are formed in electrical devices. For example, predetermined electrode patterns are formed on the top and bottom surfaces in solar cells. In addition, predetermined wiring patterns are formed on a substrate in wiring boards. Moreover, patterns of barrier ribs for separating each dot, address wires, electrodes and so on are formed in plasma display panels (PDP).
The ability to easily produce patterns having a high aspect ratio is an important issue common to these applications.
With respect to the electrode patterns of solar cells, it is preferable to reduce the surface area occupied by the electrode pattern on the light-receiving surface in order to increase the light receiving surface area of the solar cell. However, reduction in the cross-sectional area of the electrode pattern causes a decrease in electrical conductivity of the electrode pattern. Consequently, it is desirable to enhance the electrical conductivity of the electrode pattern by forming an electrode pattern that has adequate thickness in the perpendicular direction. In other words, an electrode pattern having a high aspect ratio is preferred.
In a wiring board, the use of a fine wiring pattern is desirable to achieve high density, and in this case as well, a wiring pattern having a high aspect ratio is desirable.
In PDP, if the barrier ribs are too wide, the dot pitch becomes large and it is not possible to achieve high resolution. Consequently, it is desirable to narrow the barrier ribs and in this case as well, barrier ribs having a high aspect ratio are desirable.
An example of a method for forming a desired pattern having a high aspect ratio in these applications consists of repeatedly carrying out screen printing. In the case of screen printing, however, since there are limits on the amount of paste that can be coated in a single coating, in order to form a pattern of adequate thickness; it is necessary to repeatedly print the paste many times. In addition, in the case of printing several times, it is difficult to position the coating paste with high precision each time.
Therefore, a technique is known in which patterning is carried out using a photosensitive paste as a means of forming a thick film pattern without using screen printing (see, for example, Japanese Patent Application Laid-open No. 2003-107698). However, processes using photosensitive paste require an expensive investment with respect to the mask, developing apparatus and so on, and production costs tend to be high. In addition, since the electrically conductive or insulating fine powder contained in the paste is optically opaque, random reflection occurs within the paste composition during exposure when the coating thickness is increased, thereby preventing light from adequately reaching deep portions of the paste. Consequently, it is difficult to obtain a pattern of high aspect ratio simply by using a photosensitive paste and developing directly. Moreover, since photosensitive paste has poor storage stability, control and transport of materials is bothersome.
In consideration of these circumstances, a technique is used in which a thick film pattern is formed which does not require a photosensitive paste (see, for example, Japanese Patent Application Laid-open No. 2002-290017). This document discloses a technology in which, after forming a pattern by injecting a metallized paste into a groove pattern, and transferring the metallized paste to a ceramic substrate. In the case of forming a pattern and then transferring it to a substrate in this manner, an adhesion means is required to suitably transfer the pattern to the substrate. Examples of such adhesion means include an adhesive layer or blending-in a pressure-sensitive adhesive component into the pattern. However, in the case of using an adhesive layer, not only is there the added step of forming the adhesive layer, there is also the risk of the adhesive layer components diffusing into the pattern, as well as a decrease in electrical conductivity or other predetermined pattern properties. In addition, in the case of blending a pressure-sensitive adhesive component into the pattern, it is difficult to select a preferable pressure-sensitive adhesive component so that the required properties of the pattern do not decrease due to the addition of the pressure-sensitive adhesive component. Moreover, as is described in this document, in the case of transferring to a substrate following pattern formation, there is the risk of the pattern itself being deformed or the desired aspect ratio being unable to be obtained due to pressure applied to the pattern during transfer.